The present invention relates in general to mine roof supports.
Various forms of roof supports have been proposed and adopted. One type of support, with which the present invention is concerned, employs a floor-engageable structure, such as a sill, a roof-engageable structure, such as a bar or frame, hydraulic props mounted between the floor-engageable structure and the roof-engageable structure and operable to raise and lower the latter and a goaf shield pivotably connected to the rear of the roof-engageable structure relative to a mineral face. Levers normally connect the lower part of the goaf shield to the floor-engageable structure. A piston and cylinder unit serves to control the pivotal motion between the roof-engageable structure and the goaf shield during operation of the props and stabilizes the pivot connection between the goaf shield and the roof-engageable structure. The unit is connected to the goaf shield and the roof engageable structure to extend across the pivot connection therebetween. To prevent damage to the unit, apparatus has been known which blocks the props from the pressure supply during retraction once the unit has been almost fully extended to its maximum working stroke. This known apparatus, described in U.S. Pat. No. 4,222,686 (Bev, et al.), utilizes a control rod mounted to the piston rod of the unit and extending parallel to the unit. The control rod is guided for sliding on a device fitted to the cylinder of the unit. The control rod is provided with a pair of stops spaced apart axially of the unit and these stops actuate a pilot valve mounted to the cylinder of the unit. The pilot valve in turn controls the application of pressure fluid to the props and blocks the pressure supply when the unit is approaching its fully extended position. The motion of the control rod to operate the valve is the same as the associated stroke of the unit. This known apparatus suffers from a number of disadvantages:
For example it is not particularly compact and it is prone to damage and hence is not particularly reliable.